Signal translating apparatus



April 1940- v E. w. KELLOGG 2,197,050

SIGNAL TRANSLATING APPARATUS Filed Dec. 31, 1936 2 Sheets-Sheet 1 Zhwentor;

April 1940- E. w. KELLOGG 2,197,050

SIGNAL TRANSLATING APPARATUS Filed Dec. 51, 1936 2 Sheets-Sheet 2 Jmaentor [wad/21561 1 attorney ?atented filer. l6, 1 I

s'rn'rs ,lmso

PATENT SIGNAL TRANSLATING APPARATUS tion of Delaware Application December 31, 1936, Serial No. 118,440

12 Claims.

My invention relates to signal translating apparatus, and more particularly to a novel method of and system for automatic monitoring for sound recording, transmission, or reproducing systems; and it has for its purpose the introduction of a brief period of delay between the initial reception of electrical waves representing sound, and the employment of such electrical waves for recording, transmission, or reproduction purposes, whereby control devices may have time to operate.

In recording sound, it is often found necessary to alter the amplification between the microphone and the recording device in such a.

'mannerias to reduce the extreme variations in volume. The faint passages in a musical selection, for example, are amplified in a greater degree than the loud portions. This operation is known as compression and is necessary in order to avoid overloading the recording system or record by the loud passages and to prevent the soft passages from becoming so faint that unavoidable noises in the system become conspicuous in comparison with the recorded sound. Examples of such unavoidable noises are the socalled needle scratch, or surface noise, produced by mechanical sound records and the noise which results from dirt and scratches on photographic sound records and the inherent grainy structure of the photographic image. In the transmission of electrical sound waves over wire lines, induction from adjacent lines causes noise which makes it essential to keep the minimum current strengths above a certain value. larly in radio transmission, static and other disturbances ruin the reproduction if this drops to extremely low levels, and in sound production in auditoriums, the noises due to the presence of a large number of people set the lower volume limit for useful sound reproduction. At the same time, in all of these applications, the maximum allowable intensities are set by the over-load limits of various pieces of apparatus or perhaps by considerations of reasonable loudness limits for comfort or pleasure. Since, in concert work, conditions for utilizing wide volume range are more favorable, musicians have become accustomed to rendering their selections with very great changes in loudness, and they cannot play as well if required for any reason to seriously change their manner of rendering their musical performances.

To take care of this situation, practically all systems of sound recording, transmission, and reproduction, which involve music, employ amplifiers with variable gain or amplification, the gain or volume control being manually operated, preferably by someone who knows the musical selection well enough to anticipate at least the major changes in volume of the com- Simi- (Cl. Mil-100.1)

position as played. Even the most skilled monitoring operators are frequently taken by surprise and fail to cut down the amplification until after some sudden burst of sound has produced some unfortunate impairment of quality or spoiled a record. An automatic monitoring device can operate faster than it is possible with manual monitoring, and numerous automatic monitoring systems have been employed but they have not displaced manual monitoring for the reason that they must necessarily operate much more frequently than a good manual monitor, the operator of which can use judgment and his knowledge of the selection, and therefore the automatic device results in a very large number of occasions where its failure to perform its function in an ideal manner produces more or less distortion.

Automatic monitoring devices, in general, depend upon obtaining a current or voltage as nearly as possible proportional to the amplitude of the sound waves and causing this current or voltage to change the gain of the amplifier which is amplifying the currents to be controlled. The controlling current is best obtained by producing, in an auxiliary amplifier, audio frequency currents of suitable magnitude, rectifying these audio frequency currents, and eliminating the audio frequency components in the rectified current by means of suitable filters. The last mentioned operation is necessary because failure to eliminate such audio frequency components results in serious distortion when the rectified current is applied to the tubes of the main amplifier in such a manner as to alter its gain.

Filters which will accomplish the necessary elimination of the audio frequency components, leaving only a relatively slowly varying current which is a measure of the momentary loudness or amplitude of the audiofrequency waves, have inherently and unavoidably the characteristic of a certain degree of sluggishness, or of introducing an appreciable delay between a change in strength of the rectified current and the resulting increase or decrease in the filtered current. Thus, a sudden increase in amplitude does not result in an instant rise in the filtered current, but the latter climbs to the new value at a rate dependent on the design of. the filter. Likewise, when the audio frequency currents suddenly decrease in amplitude, the filtered current drops more. or less gradually to a new and lower value. This being the case, it is obvious that the required change in amplification in the main amplifier may not take place quickly enough to obtain the desired compression on the first few waves after a. sudden change.

It has long been recognized that some measure which anticipates the changes in the sounds t be compressed would make possible improved results with automatic compressors or monitors. In a re-recording operation, such anticipation can be secured by providing two reproducing elements which operate in sequence. Thus, in a reproducing machine employing a film record, two reproducing optical systems would be employed with two photocells, separated, for exampie, by one inch of film. There would thus be two sources of current representing the recorded sound. the one delayed with reference to the other by A th of a second. The first source would be employed to operate the volume control or ground noise reduction equipment, and the output of the second photocell would be applied to the re-recording amplifier. Any change in volume which calls for the operation of the automatic volume control would affect the controlling device in time for it to operate on the recording amplifier, just as the actual change of volume reaches the latter. The offset between the two reproducing systems would be so regulated as to just compensate for the delay in action required by the filters.

Where the operation is not one of re-recording, it is less easy to provide a time interval between the input to the controlling device and the recording amplifier. It is necessary, in some manner, to provide for a delay in the waves to be recorded, but no impairment of quality in this operation is permissible. Various methods have been proposed for this purpose. For example, an electrical artificial transmission line consisting of series coils and shunt condensers might be employed, but it is found that such a line of sumcient length to produce the desired delay requires a prohibitively large number of coils and condensers. Another method is to convert the electrical waves into sound waves which travel at a lower velocity, thus simplifying the problem of producing a delay of reasonable magnitude, and to employ a microphone to convert the sound waves back to electrical waves for recording. The objection to this method is the distortion introduced by practically all devices for transforming electrical waves into sound and vice-versa, plus the distortion which would take place in the transmission of sound waves through a tube. Sound waves might also be transmitted through other mediums than air, as, for example, through solid bodies such as coils of wire. This again introduces serious distortion.

Another method consists in making a temporary record of the sound waves and reproducing them from this record after the proper time interval. The method of recording magnetically on a hard steel wire and reproducing by means of an electromagnet in accordance with the principle of the Poulsen Telegraphone has also been considered, but here again considerable distortion and noise are produced. It would also be possible to record on wax or other material and reproduce as in an ordinary phonograph, but this method is also subject to distortion, while photo- 'graphic methods of recording require time for processing.

It has been previously proposed to form records by charging the surface of an insulating material, the charge intensity varying from point to point in accordance with the signal waves. If such a record is passed under an electrode connected to the grid of an amplifier tube, the induced potential of the electrode will vary in accordance with the surface charge, and the plate current of the vacuum tube will vary in accordance with the waves. So far as I have been able to learn,

however, no use has been made of any electrostatic method of recording since it is quite unsuitable for any ordinary purpose on account of lack of permanence, and on account of the large area required for any appreciable amount of recording. These objections, however, do not apply to the specific purpose of providing a brief delay. It is therefore one of the purposes of my invention to provide the desired delay for purposes of monitoring, volume control or ground noise control, by means of an electrostatic record.

Furthermore, although the fundamental conception of an electrostatic record in the abstract may have been described, practical means of producing the record and then of erasing it so that the surface will again be in position to receive new recorded material has not, to my knowledge, been developed. It is therefore a further object'of my invention to provide effective means of charging the surface of a record in accordance with the waves to be recorded, of reproducing the waves from said record and of discharging the surface to a'specified uniform potential such that it will accept a new recording without distortion.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, will best be understood from the following description of several embodiments thereof, when read in connection with the accompanying drawings, in which Figure 1 is a diagrammatic view of one form of my invention,

Figures 2 to 7, inclusive, are similar views, respectively, of six other modifications of my invention,

Figure 8 is a detail view illustrating the angular displacement of the several light beams of the modification of my invention illustrated in Figure 7, and

Figure 9 is a fragmentary, detailed perspective view showing the relation of certain of the electrodes of the system of Figure '7.

Referring more specifically to the drawings, I have shown, in Figure 1, an embodiment of my invention in which the record consists 01' a cylindrical member I so mounted that it can be rotated at uniform speed about its axis and can be completely enclosed in evacuated space. It is well known that mechanical movements within a shielded chamber can be produced either by enclosing a complete motor inside the space and carrying the leads through suitable gas-tight insulators or by an arrangement of the nature of a synchronous or induction motor, a rotating field being produced by magnetic windings outside the chamber. The surface la of the cylinder i of Figure 1 is of insulating material so that a charge produced at one point thereon will remain deflnitely localized and not spread to adjacent areas. At the same time, the cylinder is preferably not composed entirely of insulating material but rather of a thin layer of insulating material la. over a conductive body or layer lb. This construction is preferably since it increases the electrostatic capacity of a unit of surface, thereby causing it to hold a larger charge and therefore reducing the danger of serious changes of potential due to leakage. The large capacity of the surface will also make its potential drop less when it passes under a reproducing bar 2, presently to be more fully described, than would be the case if its-capacity were less.

In the making of the record, the surface of the cylinder l is charged by means of a special electron device 3 consisting of electron emitting cathode 4, a control grid 6 and a plate 6 having a slit 1 therein. The electrons emitted by the cathode 4 are accelerated by the positive potential on the plate d, but the magnitude of the electron stream is governed by the potential of the grid 5, this potential being altered from moment to moment by the input circuit represented by the transformer 8. Most of the electrons which travel toward the anode 6 strike this anode and are conducted back to the cathode 4, but those which are directed toward the slit 1 pass through and charge that portion of the record surface which is at the moment under the slit 1. Thus, the potential to which the surface element Ia becomes charged depends on the strength of the electron stream at the moment. The slit 1 is parallel to the cylinder axis and is narrow compared with the shortest wave lengths to be recorded.

The record is assumed to be rotated in the direction indicated by the arrow A. As the charged surface la moves, it comes under the reproducing bar 2 which is also parallel to the axis and very close to the surface la. The potential changes of the bar 2 induced by the charges of the adjacent cylinder surfaces In are applied to the grid Illa of an amplifying tube ll. The bar 2, like the slit I, is narrow compared with the shortest wave length to be recorded. In order that fringing electrostatic fields may not cause a relative loss of high frequency in the reproduction, it may be desirable to place grounded bars 9 and 9' on either side of and very close to the reproducing bar 2. The grounded bars 8 and 9' will shield the reproducing bar 2 from the potential of nearby elements of the cylindrical surface which are not directly under the reproducing bar 2. The delay between the instant of recording and reproducing obviously depends on the speed of rotation of the cylinder l and the angle between the recording slit 1 and the reproducing bar 2.

After reproduction, the surface must discharge to such a potential that it will not repel elecirons which are projected toward it by the recording device 3. This means that the surface la must approach the slit I with suflicient positive charge so that the maximum electron current reaching it through slit 1 will not make it negative with respect to the cathode 4 of the recording element 3. It would be conceivably possible to discharge the record surface la by causing it to pass under a contacting brush or series of brushes which are maintained at the desired potential. This method is not desirable since it involves wear, and possibly failure to make contact with all portions of the area. Another method of discharging the record surface consists in producing within the region where discharge is to take place an ionized atmosphere, a sufliclent number of positive ions being produced to render the space conducting, so that the record can reach potential equilibrium with an adjacent electrode. If this method is em ployed, it would be necessary that the shielded uompartment in which the record runs shall contain a certain amount of gas from which the positive ions can be formed. This would not prevent the functioning of the charging device 3 since it can be operated at low enough voltages to avoid local production of positive ions, while the ions produced in the discharge device can be practically confined to the locality of the latter.

A preferred form of discharging device according to my invention is'shown enclosed within a dot-dash line H in Figure 1, and includes a cathode II, a screen or anode I l, and a grid l4 provided with end extensions II and I. The screen If is located near the cathode i2 and is at high enough positive potential to result in a large amount of ionization. The electrons emitted by the cathode II are, for the most part. collected by the anode I), but the positive ions are drawn toward the cylinder and pass through the meshes of the grid l4 to strike the surface of the record. The final potential to which the record surface is discharged will depend on the potential of electrode l4, a condition of approximate equilibrium being achieved if there are a V sufficient number of'positive ions as well as electrons present. In order that the ions may not diffuse excessively into other parts of the chamber and cause leakage or interference with the operation of the recording and reproducing device, the grid I4 is provided with the sheet metal extensions 15 and I! which practically enclose the space occupied by the discharge device.

Although the desired discharge of the record surface may be accomplished by production of positive ions, a preferable method is to cause the record surface itself to give of! electrons. This can be accomplished in either of two ways. The record surface Ia may be so treated as to make it photo-sensitive, in which case it will emit electrons when illuminated. Figure 2 shows an arrangement designed to operate according to this principle. Close to the record surface la is a grid II which is maintained at approximately the potential to which it is'desired to discharge the record surface. A lamp I. with a reflector or condensing lens i9, if desired, is used to strongly illuminate the portion of the record which is being discharged. Electrons emitted from the record surface will travel to the grid ll, the condition of equilibrium being established if there is abundant light, for as soon as the surface has a lost a suflicient number of electrons to make it slightly negative with respect to the grid H, the electrons will return to the cylinder surface as fast as they are emitted. The recording device in Figure 2 is assumed to be the same as in Figure 1.

Another method of controlling the potential of the record surface la is illustrated in Figure 3 as applied to the erasing device. The recording in Figure 3 is by the identical methods shown in Figures 1 and 2. The record shown in Figure 3 is assumed to have had its surface treated in such a manner that, when it is bombarded by electrons of suiiiciently high velocity, it will emit secondary electrons in considerably greater number than the number of primary electrons with which it is bombarded.

Reproduction in Figure 3 is the same as in Figure 1 but the method of discharge is by employment of secondary electron emission. A discharging device represented by the dot-and-dash line 2| is provided with a cathode 22 which is maintained at high negative potential and the electrons which it emits strike the cylinder surface ia at high velocity, thereby causing the emission of a large number of secondaries. These are drawn to the grid 23 of the discharging device 2l provided the latter is positive with respect to the record surface, or they will return to the record surface if the latter is positive with respect to the grid. Thus the record surface will attain a potential dependent on that of the electrode 23. By connecting the electrode 23 to a terminal of a battery or other source of voltage, the final potential of the record surface can be controlled so that it will be several volts positive with respect to cathode 4, under which condition it will not repel the electrons of the recording beam, but on the other hand will not be struck by them hard enough to give of! secondary electrons.

Methods of making a surface la photo-sensitive and also capable of emitting secondary electrons in large numbers, while maintaining its insulating properties, have been developed in connection with the Iconoscope employed in television transmission. Similar technique may be employed in treating the surface of the cylinder l for the electrostatic record heretofore described.

The recording system indicated in Figures 1,

2 and 3 is such as to cause the record surface to receive electrons. The erasing process must therefore be such as to permit the surface to either lose electrons or receive positive ions, thus leaving it positively charged. An alternative method is to cause the record surface to lose electrons in the recording operation but receive them in the erasing process. The erasing in this ,case may be accomplished by simply providing an electron emitting surface or cathode of adequate area close to the record surface. In Figure 4, the cathode 25 of an erasing device represented by the dot-and-dash line 24 performs this function. Figure 4 also shows a method of making the record in which the surface is assumed to be capable of emitting secondary electrons. This permits current to flow from the surface when it is bombarded with high velocity primary electrons but not otherwise. In Figure 4, the cathode 21 of a recording or a charging device represented by the dot-and-dash line 26 is at high negative potential and high velocity electrons strike that portion of the cylinder surface which is behind the slit 1 in the anode 28. Secondary electrons are emitted but these do not continue to flow after the cylinder element has reached a potential practically equal to that of anode 28. Therefore, if the potential of the latter is changed by the signal to be recorded while the cylinder revolves, an electrostatic record results.

Another method of recording employs a recording device like that shown in Figures 1, 2 and 3. but the cathode 4 is maintained at a sufficiently high negative potential with respect to that of the record surface such that the latter gives off secondary electrons in proportion to the number of primaries which strike it. The secondary electrons are drawn away to the electrode 6 which is maintained positive with respect to the record surface- Since the number of secondary electrons exceeds the number of primaries, the more of these strike the record surface, the more electrons the latter loses and the more positively charged it becomes. The strength of the primary beam of electrons is varied by means of the control grid 5 in accordance with the waves to be recorded.

Figure 5 shows an optical method of recording. An optical system 30 produces a beam of light which illuminates an element of the cylinder at 3i. The light intensity is caused to vary in response to the input signal. Many forms of optical systems have been developed for making photographic sound records and the optical system for an electrostatic record can be based on the same principle. In general, such light modulating devices consist in a lamp 3!, a modulating device 33 which might be a light valve, 0. Kerr cell or a galvanometer, and a lens system or optical barrel 34 which is designed to produce a narrow line of light transverse to the direction of movement of the record I. Close to the illuminated element 3| is an electrode I5 maintained at positive potential, which will draw away all of the electrons which the beam of light causes the record surface to emit. The record is erased in Figure 5 by means of a cathode or electron emitting device 36, as described in connection with Figure 4.

In the forms of the invention described above, the record (which is described as cylindrical in form) is assumed to rotate at uniform speed. It is obvious that instead of having the record rotate, it is possible to cause the recording, reproducing and erasing elements to move with respect to a fixed surface. One possible way of accomplishing this would be by employment of apparatus similar to that now used in the television Iconoscope. Figure 6 shows a device operating on this principle. The evacuated tube 38 contains a screen 39 which is so constructed that current can pass from an element on one surface to a point directly opposite on the other side, but cannot pass from one part of the screen to the other. In other words, all chargers are localized on the screen. Such screens have been constructed by coating wire gauze with an insulating enamel and placing conducting material in the meshes of the gauze, the material being subsequently treated to give it the desired properties. The recording is accomplished by an optical system which throws a beam of light of variable intensity on the screen and causes the beam to rotate in a circle on the screen. An optical system for producing a modulated light beam focused on the screen is indicated at 40 and rotation of the light beam is accomplished by causing a prism 4| to rotate about the optical axis. As the light beam travels around a circle, the illuminated elements of the screen become positively charged by emission of electrons from the sensitized surface, these electrons being drawn 01f to a grid 42 which is maintained positive with respect to the screen 35.

Reproduction is accomplished by scanning by means of a sharply focused beam of electrons which is caused to rotate in a circle on the opposite side of the screen from the light beam. The electrons emitted by the cathode 43 of the tube 38 pass through a small orifice 44a in a diaphragm 44 and are focused by theelectrodes 45 to produce a sharp spot on the screen 39. This beam is deflected vertically and horizontally by the magnet coils 46 and 41 in such a manner as to make it travel in a circle. This requires that the magnetic fields produced by the coils 46 and 41 be of equal strength and excited sinusoidally in quadrature by any well known means. The excitation of coils 46 and 41 is made to have the same frequency as the rotation of the prism 4|, so that a fixed relation is preserved between the rotation of the recording light beam and the scanning cathode ray beam. In order to make it less difiicult to insure the cathode ray beam reaching the same elements which were charged by the light beam, the cathode ray beam may be caused to oscillate at a supersonic frequency in a radial direction. This may be accomplished by impressing high frequency voltage on an electrode 48. Other means of accomplishing the same purpose are well known to designers of cathode ray tubes. Close to the screen 3| is an electrode 0, which is maintained at a suitable average potential with respect to screen (in general. a few volts negative compared with the electrode 42). When the cathode ray beam strikes a sensitive element of the screen 39, this element receives electrons and also emits secondary electrons. The presence of the latter causes it to come into voltage equilibrium withthe nearby screen 49. This discharges the element of screen 39 which was previously charged by the action of the light beam. Since the charge is transferred to the electrode 49, the latter will undergo a change of potential which may serve as the input to a vacuum tube 50. In this arrangement, thescanning of the record and discharge, or erasure, occur simultaneously. It is not possible to utilize changes in potential of the metal screen upon which the electrode 39 is built, because this is afiected at the time of recording as well as at the time of reproduction.

Instead of scanning the charged recording by means of a cathode ray beam, the reproduction can be accomplished by scanning with a second light beam. This method would have the advantage that a more sharply focused scanning beam could be produced- In Figure 7, there is shown an evacuated chamber H in which is mounted a photo-sensitive screen 52 constituting a record surface. At the end of the chamber opposite the screen is an optically fiat window It through which light from an optical system 54 enters. In this arrangement, several light beams must rotate and this could most readily be accomplished by causing the whole optical system It to revolve. Altogether, five light beams I5, I, 51, El and II enter the container ii. In Figure 7, the radial positions of these beams are indicated as if they were all in the same plane, while their relative angular positions are illustrated in Figure. 8. The modulated light beam which produces the record is shown at 55, a scanning beam (unmodulated but traveling in the same circle) is shown at 56, and 61 is an erasing beam which illuminates not the screen itself but an adjacent photo-sensitive element which, at the moment, is at a negative potential with respect to the screen 52 and emits electrons which discharge the nearby portion of the screen 52. The beams 58 and 59 are commutating beams the purpose of which will be presently-explained.

Close to the circle on the screen 52 where the sound is recorded is a segmented ring 60 which is connected to radially segmented grids 6|, the segments being mounted on suitable insulators. Directly behind the segmented grids are three rings 82, 63 and 6|, the surfaces of which are photo-sensitive, as are also the segmented grids 6i. The commutating beams of light 58 and 59 are sufliciently broad and distributed to produce no disturbance as they move from one segment a to the next, always covering at least two adjacent photo-sensitive electrode elements. Since these beams play on two adjacent photo-sensitive electrode elements, they cause these elements to assume the same potential. Thus, the beam 51 causes the segment on which it strikes to assume the same potential as the ring 62. The ring 62 is maintained at the potential at which it is desired to discharge the screen prior to recording. This is the most negative of the three rings. At thesametimethatthelightfromthebeam S1 illuminates the grid segment II and the ring 02, a portion of the same beam illuminates the corresponding segment of ring It, which is conductively attached to the grid segment. The ring segment then emits electrons in sumcient number to discharge the nearby portions of the record surface 52.

When the recording beam strikes an element of the screen surface 52, the latter gives oil electrons which are drawn away to the nearby segments of the ring BI. In order that these segments may be sufliciently positive in respect to the screen 52 to carry away the electrons, the light beam 58 is caused to illuminate the attached grid segments directly over the ring 63 which is maintained at a potential somewhat positive with respect to the ring 62. When the scanning beam 56 illuminates a charged element of the screen 52, additional electrons are given off in sufllcient number to equalize the voltage between the said element and the adjacent segment of ring 60. At the same time, the action of the commutating beam 59 has served to effectively connect this and nearby segments to the ring 64 which is positive with respect to the other rings. The screen element which constituted a portion of the record thus emits enough electrons through the action of the scanning beam 56 to bring it to a potential equal to that of the ring 64, but since the initial potential of the successive elements varies, the total charge transfer likewise varies with the recorded sound. The result is that the current flowing to the ring 64 is modulated in accordance with the charges on the elements being scanned. This current fiows through a grid leak 66 and causes changes in potential of the grid 61 of an amplifier tube 68 the output of which transmits the reproduced sound.

Although I have shown and described several modifications of my invention, it will be readily apparent to those skilled in the art that it is susceptible of many other forms and that many changes and modifications thereof may be made even with respect to the specific modifications described herein. I desire, therefore, that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In signal translating apparatus, the combination of means providing an evacuated chamber, a cylindrical electrostatic record carrier in said chamber, a recording element and a reproducing element associated therewith, said record carrier and said elements being relatively movable along an endless path about the axis of said cylindrical carrier, and said reproducing element being displaced from said recording element less than 360 along said path.

2. In signal translating apparatus, the combination of means providing an evacuated chamber, a cylindrical electrostatic record within said chamber rotatable on its axis, and a recording element and a reproducing element associated therewith, said elements being stationary and located less than 360 from each other along a curve in which the surface of said record moves.

3. The invention set forth in claim 1 characterized by the addition of an erasing element interposed between said recording and reproducing elements in a position such that it erases the signals recorded by said recording element subsequent totheir reproduction by said reproducing element whereby the recorded portion of said record is free to be recorded upon by said recording element upon each revolution thereof.

4. The invention set forth in claim 1 characterized in that said record includes a surface member of insulating material, and characterized further in that said recording element is adapted to charge said member electrostatically in accordance with the signals being recorded and said reproducing element is responsive to said electrostatic charges.

5. The invention set forth in claim 1 characterized in that said record includes a surface member of insulating material backed by a member of conductive material, characterized further in that said recording element is adapted to charge said member electrostatically in accordance with the signals being recorded and said reproducing element is responsive to said electrostatic charges, and characterized further by the addition of a discharging element interposed between said reproducing and recording elements in a position such that it discharges said signal charges subsequent to reproduction thereof by said reproducing element, whereby to render the record surface member free to successively receive new signal charges from said recording element.

6. The invention set forth in claim 1 characterized in that said record is stationary and includes a surface member of insulating material, characterized further in that said recording element and said reproducing element .are movable relative to said member along said endless path, and characterized still further in that said recording element is adapted to charge said member electrostatically in accordance with the signals being recorded and said reproducing element is responsive to said electrostatic charges.

7. The invention set forth in claim 1 characterized in that said record is stationary and includes a surface member of insulating material, characterized further in that said recording element and said reproducing element are movable relative to said member along said endless path, characterized still further in that said recording element is adapted to charge said member electrostatically in accordance with the signals being recorded and said reproducing element is responsive to said electrostatic charges, and characterized still further by a discharging element interposed between said'recording and reproducing elements, said discharging element being movable with said recording and reproducing elements and being so located with respect thereto that it discharges said signal charges subsequent to reproduction thereof by said reproducing element whereby to render said record surface member free to be successively charged by said recording element with new signal charges.

8. In apparatus for making and reproducing an electrostatic record, the combination of an evacuated chamber, a record surface therein adapted to receive electrostatic charges, a source of electrons, means for directing an electron beam emitted by said source onto said record surface, means for modulating said electron beam in accordance with signals to be recorded whereby said surface may be electrostatically charged in accordance with said signals, a reproducing electrode adjacent to said record surface responsive to the signal charges thereon, and means for erasing said charges and rendering said surface free to be charged anew in accordance with further signals being recorded.

9. Apparatus for making and reproducing an electrostatic record of sound waves comprising, in combination, a record having a surface adapted to receive electrostatic charges, means for causing a beam of electrons to impinge upon an element of said surface whereby to charge said surface, means for modulating said beam in accordance with the sound waves being recorded, reproducing means responsive to electrostatic charges, said reproducing means being located adjacent another element of said record whereby it is subject to be affected by the potential of said second-named element, means including an electrode for discharging said surface, and means for rendering the space between said record and said electrode electrically conducting.

10. Apparatus for making and reproducing a record of sounds comprising a photo-sensitive screen having a plurality of elements which are insulated from each other, recording means comprising a beam of light modulated in accordance with the sounds being recorded, means for directing said modulated light onto certain elements of said screen, means for drawing away electrons emitted from the elements illuminated by said recording beam,'reproducing means comprising an electrode so constructed and arranged as to have the potential thereof selectively affected by an element of the record surface, and discharging means whereby the screen surface, after reproduction by said flrst-named reproducing means, is uniformly discharged to an approximately controlled potential.

11. In apparatus for making and reproducing an electrostatic sound record, the combination with a record having an insulated surface, of means for charging successive elements of said record surface to potentials dependent on the sound waves to be recorded, reproducing means comprising an electrode to which elements of said surface are successively discharged, amplifier means for amplifying the electrical impulses corresponding to fluctuations of potential of said reproducing electrode, and recharging means including an electrode spaced from said record surface whereby portions of said record surface subsequent to reproduction are discharged to a uniform potential, said recharging means comprising means for rendering conducting the space between said record surface and said last-named electrode.

12. In signal translating apparatus, the combination of an electrostatic record carrier including a surface member of insulating material backed by a member of conductive material, a recording element adapted to charge said lastnamed member electrostatically in accordance with the signals being recorded, a reproducing element responsive to said electrostatic charges, said record carrier and said elements being relatively movable along an endless path and said reproducing element being displaced from said recording element less than 360 along said path, and a discharging element interposed between said reproducing and recording elements in a position such that it discharges said signal charges subsequent to reproduction thereof by said reproducing element'whereby to render said record surface member free to successively receive new signal charges from said recording element.

EDWARD W. KELLOGG. 

